The present invention relates to the high voltage blocking characteristics of power diodes that also conduct large currents under forward bias. Power diodes are used in multiple applications that require rapid switching between a voltage blocking state that allows no current to pass and an on-state that quickly allows large currents to flow in one direction. The problem addressed in this invention is that of achieving a device that operates optimally in both states.
Current technology includes power Schottky diodes that have doped Junction Barrier Schottky (JBS) structures to improve the blocking characteristics of the diode. Statutory invention registration No. H40 (Buchanan 1986) discloses a silicon power diode that includes field shields within the Schottky barrier of the device. The field shields are highly doped p-type diffusions in the n− type silicon drift layer directly adjacent the metal-silicon heterojunction. Buchanan, however, is only concerned with increasing the reverse blocking voltage that the Schottky diode can withstand. These p− type diffusion regions have the possibility of increasing the forward voltage drop in the on state. Buchanan offers no disclosure in regard to maintaining forward conductivity in the presence of voltage blocking enhancements.
U.S. Patent Application Publication No. 2006/0237813 (Hashieh) explains the limitations of the Buchanan disclosure and sets forth an attempt at addressing the forward biased performance problems inherent in a Schottky diode with JBS implants in the drift region. Hashieh forms an array of PN junctions as adjacent diffusion regions to reduce forward barrier heights while increasing reverse blocking voltages. Hashieh's array is located within the drift region of the device and includes highly doped p+ and n+ implants directly adjacent a top silicide layer to which a metal contact is applied. The n+ implants function as forward barrier reduction regions while the p+ implants function as the opposite—blocking voltage increasing regions. The silicide layer in the Hashieh device increases forward conductivity as well.
Other publications also the show a JBS structure in the drift region of Schottky power diode. U.S. Pat. No. 6,104,043 (Hermansson et al., 2000) discloses a Schottky diode in which the JBS implants are positioned within a silicon carbide drift layer. U.S. Pat. No. 6,524,900 (Dahlqvist, 2003) improves the method of controlling the temperature dependence of a junction barrier Schottky diode, particularly in materials having conduction bands exceeding 2 eV.
Forming Schottky barrier diodes with silicon layers has been known for years. Adding JBS technology to a silicon diode has already been shown, for example, in European Patent Application No. EP 0372428 (Sugita 1990). Prior research in power diodes, including a previously published article from the Journal of Electronic Materials (Henning 1998), also discloses the properties of forming a heterojunction between a silicon mesa and a semiconductor drift region of silicon carbide. The prior work that incorporates JBS structures in silicon layers fails to solve the problem, however, that increasing on-state performance decreases the maximum voltage blocking capability of the device.
Another combination of JBS technology with silicon contact layers was disclosed at the International Symposium on Power Semiconductor Devices and Integrated Circuits (Tanaka et al. 2005). Tanaka discloses an example of a prior art diode using both a silicon layer adjacent the anode and a JBS structure in the drift layer to form termination regions at the edges of the device. The JBS implanted structures protect the Tanaka diode from high electric fields along its edges and cross currents in that region. Tanaka fails to realize, however, the possibility of using JBS technology to improve the reverse biased characteristics of the diode by incorporating the JBS implants within the conductive channel of the drift region. Tanaka also uses a standard silicon carbide substrate that is bound to increase the on resistance of the device.
Accordingly a need still exists for an improved power Schottky diode having desirable operating parameters in both the on-state and under a reverse bias.